This invention relates to devices exhibiting a variable transmittance of light and radiant heat. More particularly, it relates to those structures which transmit light and radiant heat at one temperature, but which become less permeable to the transmission of light and radiant heat when exposed to a second, preselected temperature.
In bright sunlight, closed edifices made primarily from glass and buildings with glass windows experience heat buildup due to transfer of heat from the sun through the glass via radiation. Heretofore, glass has been coated by some means to prevent this heat transfer. For example, greenhouses, especially the roof areas, are generally sprayed with a white pigment thereby reducing light transmission and radiant heat buildup. Application of the white pigment is difficult, must be repeated at regular intervals and leaves an aesthetically undesirable appearance. More importantly, although light and radiant heat transmission may be desired at various times, pigmentation permanently reduces the transmission of heat and light.
More recently, glass has been tinted with dyes, etc., or has been coated with a reflective-transparent solar control film. For example, U.S. Pat. No. 3,290,203 describes a solar control film consisting of a composite of a thin layer of metal, such as aluminum, deposited on a thin transparent plastic film. The composite is attached to a glass window by a water-activated adhesive, to reduce ultraviolet, infrared and visible light transmission and heat radiation. While such solar control films eliminate the aesthetically undesirable effects of pigmentation, they are generally more expensive. Furthermore, the usefulness of such solar control films is reduced by the pressure of unattractive gas bubbles in the water-soluble and pressure-sensitive adhesives normally used to adhesively bond the composite to the glass. Heat-sensitive adhesives have been found that eliminate this problem, however, the adhesives have a short shelf life making their utilization impractical. Moreover, like pigmentation, the application of transparent reflective films to glass renders the window permanently reflective, thereby reducing light and radiant heat transmission even when transmission of substantial amounts of heat and light are desired. Tinted glass eliminates the application problem experienced with the solar control film. As with pigmented glass, however, the partial transparency of the tinted or dyed glass is the same regardless of the weather conditions.
Plastic films such as high and low density polyethylene, polyester, ethylene/vinyl acetate copolymer, polyvinyl chloride, polybutylene, polyvinyl fluoride and polycarbonate are being employed as envelopes for inflatable structures and are replacing glass as greenhouse covers. The plastic films are often made permanently translucent to prevent excessive light transmission and radiation heat buildup. As with the aforementioned methods, such plastic films exhibit partial transparency which does not change with variations in the environmental conditions such as temperature and light.
In view of the aforementioned deficiencies of the prior art devices, it would be highly desirable to provide an energy saving device which has the ability to reduce light transmission and heat radiation when the conditions warrant, i.e., periods when the sun is bright and the temperature high, but which permits full light and heat transmission at other times when temperatures are lower.